Systems for checking the operability and seal tightness of fuel tank systems are required by law in some countries and are, for example, known from the German patent specifications DE 101 33 823 C2 and DE 196 36 431 B4. In the case of a tank leakage diagnostic module (diagnosis module tank leakage, DMTL), the fuel tank or the subspace to be checked can, for example, be pumped up with gas or a gas mixture using an electric air pump. A current profile of the air pump is ascertained and evaluated. A leakage can be suggested from this current profile.
In this procedure, a reference opening and a 3/2 directional control valve can be used. The 3/2 directional control valve is initially switched in such a manner that the air pump delivers air across the defined reference opening. Said 3/2 directional control valve is subsequently switched in such a manner that the air pump delivers air into the fuel tank. If, in so doing, the air pump builds up the same or a higher pressure than across the reference opening, the current consumption of the air pump is also the same or higher than when pumping across the reference opening. In this case, the conclusion can be drawn that a substantial leak is not present at the fuel tank or respectively at the subspace to be checked. If, on the other hand, the air pump achieves a lower current value than when pumping across the reference opening, a leakage in the space being checked can be inferred which is greater in size than the reference opening.
Such a tank leakage diagnosis is usually performed in the after-run period, i.e. after turning off the vehicle. To this end, the control device of the vehicle must also continue to operate after the vehicle has been turned off. This leads to a load on the onboard power system. Different effects on the air pump can furthermore influence the current profile ascertained. In certain circumstances, this can lead to a faulty diagnosis.
An alternative known method for tank leakage diagnosis is based on the evaluation of the outgassing behavior of the fuel in the fuel tank. In this method, an interpretation of the measurement results can also be affected by outside influences. For example, a swashing of the fuel in the fuel tank and a strong outgassing can lead to a faulty diagnosis.